1. Field of the Invention
The present invention relates to deflecting diaphragm pressure transducers.
2. Description of the Prior Art
U.S. Pat. No. 3,793,885 to Roger L. Frick for a Diaphragm Construction For Differential Pressure Transducer shows a diaphragm that is deflected under pressure and has a central disc deflecting or sensing portion, an outer supporting ring clamped in a housing and an isolation web extending between the support ring for the diaphragm and the central disc deflecting portion. The web is configured to make the center portion of the diaphragm act like a free edge diaphragm by reducing the bending moments of the outer edges where the diaphragm is supported. In particular, the form shown in FIG. 3 of U.S. Pat. No. 3,793,885 has found wide acceptance in the field, and the action of the reduced section web of the diaphragm is reliable and minimizes hysteresis and instability.
Particularly, in relation to differential pressure sensors, the use of the diaphragm deflecting relative to a pressure sensor housing surface, and capacitive sensing of the diaphragm has been found to be useful as has been taught in U.S. Pat. No. 3,793,885. In the sensor, the housing surface that is adjacent to the diaphragm, and toward which the diaphragm deflects, has a concave surface shape conforming across its diameter to the deflected surface shape of the diaphragm under pressure. A thin stationary capacitive plate is formed on the adjacent concave housing surface toward which the diaphragm deflects, as shown in U.S. Pat. No. 3,793,885, is positioned to substantially fully support the diaphragm when the diaphragm is subjected to its maximum rated overpressure, so that overpressures will not destroy or damage the diaphragm. The diaphragm is thus supported fully at its center on the concave stationary capacitor plate and housing surface under such overpressure.
The sensing diaphragm central portion must be sensitive enough to provide useful signals when pressure differentials are only one or two psi, but because the line pressures may be several hundred psi, extremely high overpressures can be experienced. The support for the diaphragm on the housing prevents damage to the diaphragm under such high overpressures.
It has been found that the web portion or reduced thickness portion that connects the outer mounting ring of the diaphragm to the central portion becomes more highly stressed than the central portion of the diaphragm, as pointed out in U.S. Pat. No. 3,793,885. The overall performance of the diaphragm construction shown in U.S. Pat. No. 3,793,855 is considerably improved over a uniform thickness diaphragm due to the isolation of the stresses from the mounting ring and the resulting minimization of hysteresis and instability of the diaphragm.
The present invention improves the performance of the diaphragm construction under field operation further, by controlling and reducing the maximum stress in the web between the outer diaphragm support ring and the center plate. The diaphragm using the web is not stretched or stressed in its rest position.
U.S. Pat. No. 4,370,890 also shows a sensor having a housing configured to support a stretched diaphragm, and also using a concave or recessed surface in the housing to support the diaphragm in deflected position.